1. Field of the Invention
The present invention relates to a lithography system employing a charged-particle beam such as an electron beam. More particularly, this invention is concerned with a technology useful in preventing deterioration of a chip having apertures set in an array in an electron-beam lithography system adopting a blanking aperture array (BAA) lithography method. The apertures are used to pattern an exposed sample (specifically, a wafer).
2. Description of the Related Art
Charged-particle beam lithography systems employing a charged-particle beam such as an electron beam are attracting attention as high-resolution lithography systems superior to photolithography systems. Compared with a photolithography system, the charged-particle beam lithography system has the advantage of being able to draw a high-resolution pattern. However, the charged-particle beam lithography system has the drawback of low throughput. Various methods such as a block lithography method and the BAA method have been proposed in efforts to improve throughput. The present invention relates to a BAA type charged-particle beam lithography system. A BAA type electron-beam lithography system has been disclosed in U.S. Pat. No. 5,256,579 or the like.
According to the BAA method, a BAA chip or a chip having numerous fine apertures arrayed therein is positioned on the path of a charged-particle beam. The charged-particle beam is thus split into numerous fine beams. The BAA chip has deflecting electrodes formed on the surface thereof in association with the apertures. When no voltage is applied to the electrodes, the fine beams passing through the apertures are irradiated as they are. When a voltage is applied to the electrodes, the fine beams passing through the apertures are deflected and intercepted. Thus, the on and off states of the fine beams passing through the apertures can be controlled individually. The numerous beams delivered by the BAA chip are swept undirectionally over a sample. Fine beams to be used to expose part of the sample are turned on or transmitted. Consequently, one point on the sample is exposed many times. High exposure energy can therefore be exerted. Moreover, the rate of an increase or decrease in the number of fine beams to be turned on or off becomes moderate. This brings about the advantage that a focal point can be adjusted readily finely.
In a BAA type charged-particle beam lithography system, a charged-particle beam emitted from a charged-particle source is irradiated directly to a BAA chip. This poses a problem in which an unopened part of the BAA chip is thermally damaged. To be more specific, since the beam is kept irradiated to the unopened part, the part is locally heated or impurities adhere to the part. Consequently, the pattern of apertures of the BAA chip deforms. Moreover, melting of the BAA chip, or especially, melting of deflecting electrodes may occur. Eventually, the BAA chip deteriorates and the service life thereof is shortened.
For overcoming the above drawbacks, for example, an accelerating voltage can be, conceivably, limited in order to reduce the magnitude of a current (the magnitude of a charged-particle beam) to be supplied. The accelerating voltage is applied to the cathode and anode of a charged-particle beam source. The intensity of the beam irradiated finally to a wafer is lowered accordingly. This poses another problem in that, since the irradiation time per one shot must be extended in order to draw a pattern on the wafer, throughput decreases.
Moreover, it is conceivable that the lithography system is used with the magnitude of a current reduced. However, if the use is extended over a prolonged period of time, the unopened part of the BAA chip would be thermally damaged. Eventually, a technique in which the magnitude of a current is reduced cannot be said to prove effective.
An object of the present invention is to provide a charged-particle beam lithography system capable of preventing deterioration of a BAA chip without a reduction in the magnitude of a charged-particle beam used for exposure. The charged-particle beam lithography system can eventually contribute to maintenance of high-precision exposure and to extension of the service life of the BAA chip.
For overcoming the foregoing drawbacks of the related art, according to the present invention, a mask having apertures and matched with a BAA chip is placed in front of the BAA chip. The magnitude of a charged-particle beam to be irradiated to the unopened part of the BAA chip is thus reduced.
To be more specific, according to the present invention, there is provided a charged-particle beam lithography system including a charged-particle beam emitter source, and a chip having a plurality of apertures arrayed therein. The plurality of apertures shapes a charged-particle beam emitted from the emitter source so that the cross section of the charged-particle beam will assume a predetermined shape. The charged-particle beam lithography system uses the charged-particle beam having passed through the apertures to pattern an exposed sample. The charged-particle beam lithography system further includes a mask having a plurality of apertures bored therein. The plurality of apertures is arrayed in the same manner as the plurality of apertures arrayed in the chip, and has a size that is any multiple of the size of the apertures. The mask is positioned on a path, along which the charged-particle beam travels, between the charged-particle beam emitter source and chip.
According to the configuration of the charged-particle beam lithography system of the present invention, the mask is positioned on the path, along which the charged-particle beam travels, between the charged-particle beam emitter source and chip (BAA chip). The mask has the plurality of apertures bored therein. The plurality of apertures is arrayed in the same manner as the plurality of apertures bored in the BAA chip and has a size that is any multiple of the size of the apertures. The charged-particle beam emanating from the emitter source is irradiated to the mask. Portions of the irradiated beam that have passed through the apertures of the mask are irradiated to the corresponding apertures of the BAA chip. A majority of the irradiated beam passes through the apertures and then travels downstream. Owing to the intervention of the mask, the magnitude of a beam irradiated to an unopened part of the BAA chip can be reduced without a reduction in the magnitude of the charged-particle beam emitted from the emitter source and used for exposure.
Consequently, thermal damage caused by a beam irradiated onto the unopened part of the BAA chip can be alleviated drastically. In other words, the unopened part is not heated locally and impurities do not adhere to the unopened part. Consequently, deformation of the pattern of apertures in the BAA chip can be prevented. Moreover, the drawback that the BAA chip melts can be overcome. This leads to suppression of deterioration of the BAA chip, and eventually contributes to maintenance of high-precision exposure and extension of the service life of the BAA chip.
Moreover, in the charged-particle beam lithography system, the apertures bored in the mask may have the same shape as the apertures bored in the BAA chip and also have the same size. In this case, beams transmitted by the apertures of the mask can be passed through the corresponding apertures of the BAA chip as they are. Namely, the magnitude of a beam irradiated to the unopened part of the BAA chip can be reduced further. The aforesaid advantage of the present invention will be exerted more effectively.
In the charged-particle beam lithography system, the mask may be held on a stage that is movable in the horizontal directions. For exerting the aforesaid advantage of the present invention, the mask and BAA chip are preferably positioned so that the apertures of the mask and those of the BAA chip will align with each other. However, in practice, it is very hard to accurately align the apertures of the mask with those of the BAA chip. In this case, the stage is moved in the horizontal directions. Thus, the apertures of the mask and BAA chip can be aligned with each other readily. Consequently, the advantage of the present invention can be exerted more effectively.
Moreover, in the charged-particle beam lithography system, an electronic lens may be interposed between the mask and chip. The electronic lens forms an image of the pattern of apertures represented by a charged-particle beam, which has passed through the apertures of the mask, on the chip according to the size of the apertures of the mask that is a multiple of the size of those of the chip. For exerting the aforesaid advantage of the present invention effectively, the image of the pattern of apertures represented by a charged-particle beam, which has passed through the apertures of the mask, should preferably be formed on the chip, or more particularly, on the apertures of the chip. In this case, the electronic lens is used to accurately form the image of the pattern of apertures on the chip according to the size of the apertures of the mask that is a multiple of the size of those of the chip. Thus, the aforesaid advantage of the present invention is exerted more effectively.
Furthermore, in the aforesaid charged-particle beam lithography system, an alignment coil may be interposed between the mask and chip. The alignment coil is used to align the image of the pattern of apertures with the apertures of the chip. It is easy to align the apertures of the mask with the apertures of the BAA chip by moving the stage properly in horizontal directions. The precision in alignment is not always satisfactory. In this case, the precision in alignment can be improved by employing the alignment coil. Thus, the aforesaid advantage of the present invention can be exerted much more effectively.